26 M. A. Daly — Mechanics of Igneous Intrusion. 



walls by the friction of the moving magma. On the lacco- 

 lithic theory one would expect many of the xenoliths to form 

 elongated smears in the granite rock. This is indeed occa- 

 sionally seen but most exceptionally; as a rule the xenoliths 

 have just that irregularity of form and arrangement which 

 they should have if they had been shattered oif by the hot 

 magma just before its final consolidation. Throughout its 

 long, earlier history the magma must, in every case, have had 

 a much more effective shattering power. 



It may be noted that the shattering of crystals and rock- 

 fragments, when immersed in silicate melts, has often been 

 observed.* The strains are, in such cases, necessarily of a 

 lower order than those developed on the wall of a batholith 

 where, therefore, shattering is even more certainly brought 

 about. 



Relative densities of magma and xenolith. — In his first 

 intrusion-paper, the writer published the results of his attempt 

 to calculate the possible specific gravities of the chief types of 

 molten magmas under plutonic conditions. The calculations 

 were based on Earus's well-known fusion experiments on 

 diabase. The specimen investigated had a specific gravity of 

 3-0178; when fused to a glass and cooled to 20° C, a specific 

 gravity of 2*717. He further states that the glassf showed 

 an expansion of 3*9 per cent in '•'melting" and, as glass, 

 expanded 0*000025 in volume for a temperature rise of 1° C. 

 through the interval 0°-1000° C. and 0*000047 in volume for 

 1° C. through the interval 1100°-1500°. The "melting" 

 expansion (solidification-contraction) and the varying rate of 

 expansion (or contraction) above and below 1000° C. seem to 

 show that some crystallization of the melt took place during 

 the experiment. Such crystallization was inevitable under 

 the conditions of the experiment, in which the cooling lasted 

 several hours. . Barus's curves do not, therefore, show directly 

 the volume changes suffered by pure diabase glass in passing 

 from the molten isotropic state to the rigid isotropic state at 

 room temperature. Excluding the "solidification" contrac- 

 tion, the glass loses but 3*5 per cent of its volume in passing 

 from the molten state at 1400° C. to room temperature; the 

 loss of volume through the same temperature interval was 

 calculated in the first paper as about 8 per cent. Bams found 

 that the net decrease in specific gravity in passing from rock 

 at 20 C. to glass at 20° C. was 10 per cent. For his diabase 

 specimen, therefore, the decrease of specific gravity in passing 



*Cf. C. Doelter and E. Hussak, Neues Jahrb. fur Min. etc., 1884, p. 18; 

 A. Becker, Zeitschr. d. d. geol. Ges., xxxiii, p. 62, 1881. 



f " Throughout this paper the molten rock solidifies into an obsidian." 

 C. Barus in Bull. 103, U. S. Geol. Surv., p. 26, 1893. 



